That's so awesome, I have just recently taken up quilting and this sounds like a great idea!
A few things I would suggest is to make sure to prewash and iron all of your fabrics (before cutting and piecing them together) and always use 100% cotton, non-stretchy materials only. (I would avoid t-shirt cotton fabric for the autographs, way too stretchy)
The prewashing because you are talking about using different colored materials with white fabric. Eventually you will need to wash the quilt, you don't want all of your hard work to end up bleeding into the autographed parts.
100% cotton fabric (and thread) for 2 reasons, easier to piece together because blended fabrics stretch more and are harder to work with, just trust me...this is really important. Also, when the quilt is washed the eventual shrinkage will be more consistent (even after prewashing, fabrics and thread shrink a bit).
Also, be sure to make the fabric 'paper' you have the characters sign is a good bit larger than the square will actually end up being, you will need room for pre-washing shrinkage and cutting. If you are concerned that they will take up a larger amount of space signing than you want, use masking tape or something similar to block off a specific area for their signature in the middle of the page. (if that makes sense, LOL)
I would also use 'fabric' markers (they come in all sorts of colors) rather than Sharpies. Sharpies tend to bleed and fade a bit during washing.
***your quilter friends may have already advised you of the above suggestions, but they are important***
I have not printed any fabric yet, but I eventually want to try it.
Good luck, what a great idea!!!!